DECOMPOSITION OF COMPOUNDS. 



55 



Analysis and synthesis. These expressions refer to two method 

 of research in chemistry, accomplished by two kinds of reac- 

 tions, analytical and synthetical. 



Analysis is that mode of research by which compound sub- 

 stances are broken up into their elements or into simpler forms 

 of combination, and analytical reactions are all chemical pro- 

 cesses by which the nature of an element, or of a group of 

 elements, may be recognized. 



Synthesis is that method of research by which elements are 

 made to unite to produce compound substances. 



Analytical and synthetical methods, or reactions, frequently 

 blend into one another. This means : A reaction made with 

 the intention of recognizing a substance may at the same time 

 produce some compound of interest from a synthetical point of 

 view. 



Acids. The many compounds formed by the union of ele- 

 ments are so various in their nature, that no system of classifi- 

 cation proposed up to the present time can be called perfect. 

 'Ihere are, however, a few groups or classes of compounds, the 

 properties of which are so well marked, that a substance be- 

 longing to either of them may easily be recognized. These 

 groups are the acids, bases, and neutral substances. 



Acids are characterized by the following properties : 



1. They have (when soluble in water) an acid or sour taste. 



2. They change the color of many organic substances, for 

 instance of litmus, from blue to red. 



3. They contain hydrogen, which can be replaced by metals, 

 the compound thus formed being a salt. 



According to the number of hydrogen atoms replaceable by 

 metals, we distinguish monobasic, bibasic, and tribasic acids. 

 Hydrochloric acid, HC1, is a monobasic, sulphuric acid, H 2 S0 4 , is 

 a bibasic, phosphoric acid, H 3 P0 4 , is a tribasic acid. 



Bases or basic substances show properties which are opposite 

 to those of acids. These properties are : 



1. They have (when soluble in water) the taste of lye, or an 

 alkaline taste. 



2. They restore the color of organic substances when pre- 

 viously changed by acids, for instance of litmus, from red to blue. 



